1. Field of the Invention
The present invention relates to a method and an apparatus for obtaining information on a size of particle or defect by measuring light scattered by the particle or defect in a crystal.
2. Related Background Art
There is a conventional method for measuring a distribution of defects in a crystal, as disclosed in Japanese Laid-open Patent Application No. 1-151243, in which a laser beam finely converged is projected into an object, in which image information is obtained of scattered light from defects in the object under irradiation with the laser beam, and in which a density distribution of defects is obtained by processing the image information. A scattering image for example as shown in FIG. 10 may be acquired through this method. Further, a dust counter (employing a laser scattering method) is used for detection of dust existing in liquid or in air, which pulse-counts scattered light from the dust under irradiation with a laser beam. There is also a method known to obtain a particle size distribution, using an angular dispersion of optical scattering from particles.
Using the method as described in Japanese Laid-open Patent Application No. 1-151243, an image may show a size of each particle, as shown in FIG. 10. However, if a size of particle is below one tenth of wavelength of laser beam, an image would have unsharpness or ambiguity due to halation, which makes it difficult to judge a size of particle from the size as shown in the image. Also, the image size of particle may differ depending upon a focus condition of an optical system used, as shown in FIG. 11A and FIG. 11B, which also makes it difficult to obtain accurate information about the size of particle.
The conventional laser scattering methods have a detection limit of about 0.1 .mu.m, so that a particle smaller than the detection limit cannot be detected by the methods. A plurality of fine dust particles present in a laser beam cannot be resolved in the conventional methods, limiting an improvement in detection accuracy.
When the angular dispersion method is employed to obtain the particle size, using the angular dispersion of scattered light intensity, no dispersion is observed from particles with a particle size of about one tenth of beam wavelength, failing to obtain the particle size.
In the measurement of particle size using the angular dispersion of scattered light intensity, it is normally presumed that particles have structures and sizes similar to each other. Thus, the angular dispersion method is not suitable for measurement of a sample in which particles are distributed in a range of several times to about ten times in size. In case that particle sizes are distributed in a broad range, small particles are behind large particles to become undetectable, because the scattering intensity is proportional to the sixth power of particle size.